Ra-tab array bump tab tape based I.C. package

ABSTRACT

An electronic package assembly that includes a flexible circuit wrapped around a metal substrate. The leads of the flexible circuit are connected to an integrated circuit that is mounted to the top surface of the substrate. The flexible circuit has a plurality of metal pads located adjacent to the bottom surface of the substrate. The flexible circuit also contains a power/ground plane and a number of conductive signal lines that couple the integrated circuit to the metal pads. The pads can be soldered to a printed circuit board to electrically couple the integrated circuit to the board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic package assembly.

2. Description of Related Art

Electronic packages typically include an integrated circuit that isencapsulated in a plastic or ceramic housing. The housing has aplurality of wire leads or pads that are coupled to the circuit. Thepackage is typically mounted to a printed circuit board by soldering theleads/pads to corresponding pads or plated through holes in the board.

Integrated circuits generate heat which flows into the housing. The heatis removed from the package primarily by convection from the housing, orconduction into the circuit board through the leads/pads. Plastic andceramic are relatively poor thermal conductors, wherein the heat cancreate high junction temperatures in the circuit. It is thereforedesirable to provide an electronic package assembly that is thermallyefficient in removing heat generated by an internal electrical device.

Electronic packages are typically constructed with internal signal linesthat are parallel with each other. When used with a high speed circuit,parallel lines can produce cross-talk and other electrical noise in thesignals. It is therefore desirable to have an electronic package thatminimizes the amount of electrical noise in a signal that travelsthrough the package. Additionally, it is also desirable to have anelectronic package assembly which maximizes the lead density of thepackage and the input/output (I/O) of the integrated circuit.

SUMMARY OF THE INVENTION

The present invention is an electronic package assembly that includes aflexible circuit wrapped around a metal substrate. The leads of theflexible circuit are connected to an integrated circuit that is mountedto the top surface of the substrate. The flexible circuit has aplurality of metal pads located adjacent to the bottom surface of thesubstrate. The flexible circuit also contains a power/ground plane and anumber of conductive signal lines that couple the integrated circuit tothe metal pads. The metal pads are typically soldered to a printedcircuit board, so that the integrated circuit is coupled to the board.

The flexible circuit may have multiple layers of signal lines that areinterconnected to reduce the amount parallelism and cross-talk betweenthe lines. The substrate is typically constructed from a metal such ascopper which reduces the overall thermal resistance of the package. Theassembly may also be construed so that the substrate functions as apower/ground plane. The metal pads typically would fully populate thebottom surface of the substrate, so that the package has a maximumnumber of contacts.

Therefore it is an object of the present invention to provide anelectronic package assembly that is thermally efficient in removing heatfrom an integrated circuit.

It is also an object of the present invention to provide an electronicpackage assembly that reduces the amount of electrical noise in thesignal lines of the package.

It is also an object of the present invention to provide an electronicpackage assembly that utilizes an optimum amount of leads.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will become morereadily apparent to those ordinarily skilled in the art after reviewingthe following detailed description and accompanying drawings, wherein:

FIG. 1 is a perspective view of an electronic package of the presentinvention;

FIG. 2 is a cross-sectional view of the electronic package of FIG. 1;

FIG. 3 is a side view of an integrated circuit mounted to a substrate;

FIG. 4 is a view similar to FIG. 3 showing a pair of flexible circuitsattached to the integrated circuit and bonded to a top surface of thesubstrate;

FIG. 5 is a view similar to FIG. 4, showing the flexible circuitswrapped around the substrate;

FIG. 6 is a view similar to FIG. 5, showing a plurality of solderpreforms that are then reflowed into metal pads in the flexiblecircuits.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings more particularly reference numbers, FIG. 1shows an electronic package assembly 10 of the present invention. Theassembly 10 has a plurality of pads 12 that can be soldered to a printedcircuit board (not shown). The circuit board typically contains a numberof assemblies 10 that communicate through the board. The pads 12 arepreferably arranged in a grid like pattern to maximize the pin densityof the package 10.

As shown in FIG. 2, the package 10 includes an electrical device 14which is typically an integrated circuit (IC) that contains logic,memory, etc., as is known in the art. The device 14 is mounted to asubstrate 16 and located within a cavity 18 formed in the top surface 20of the substrate 16. The substrate 16 also has a bottom surface 22 and apair of side surfaces 24.

The package 10 includes a flexible circuit 26 that wraps around thesubstrate 16. The flexible circuit 26 may include a first layer ofdielectric material 28 that separates the substrate 16 from a layer ofconductive signal lines 30. The dielectric material is preferablyconstructed from a polyimide that is flexible enough to bend around thesubstrate 16. The conductive signal lines 30 are typically copper linesetched in a predetermined pattern. The flexible circuit 26 usually has aplurality of signal lines that transmit digital signals from the IC 14to the printed circuit board.

The conductive signal lines 30 are typically separated from an outerconductive plane 32 by a second layer of dielectric 34. The conductiveplane 32 can be used as either a power or ground plane for theintegrated circuit 14. The conductive plane 32 is preferably covered byan outer layer of dielectric material 36, The flexible circuit 26 mayhave additional layers of signal lines and power/ground planes separatedby layers of dielectric. The flexible circuit 26 may also include vias38 that interconnect the different conductive signal lines and/or theconductive planes. The signal lines 30 may be constructed so that no twosignal lines run parallel along the entire length of the flexiblecircuit 26. Such a construction reduces the inductance and capacitanceof the circuit and the amount of electrical noise between the lines.Controlling the impedance of the circuit 26 is particularly importantfor high speed chips 14.

The flexible circuit 26has a plurality of leads 40 that extend from thesignal lines 30 and power plane 32. The leads 40 are bonded to pads 42on the surface of the device 14. Both the device 14 and the cavity 18are covered with a dielectric material 44. The dielectric material 44 ispreferably a plastic typically used to encapsulate electrical devices.

The pads 12 extend from the signal lines 30 and power/ground plane 32.The pads 12 couple the device 14 to the printed circuit board throughthe signal lines 30 and power/ground plane 32 of the flexible circuit26. The substrate 16 is preferably constructed from a metal such ascopper which is both electrically and thermally conductive. Thesubstrate 16 may also function as a power/ground plane, wherein aplurality of pads 12 are coupled to the substrate 16. The power/groundpads 12 also provide a thermal path for heat to flow from the substrate16 to the circuit board. The high thermal conductivity of the coppersubstrate 16 greatly reduces the temperature differential throughout thepackage 10 and the junction temperatures of the discrete elements on thedevice 14. The power/ground pads and signal pads both extend throughholes 46 formed in the power/ground plane 32 and dielectric layers. Theholes 46 typically contain a dielectric material between thepower/ground plane 32 and the pads 12 to prevent shorting between thetwo conductive members.

FIGS. 3-6 show a preferred method of constructing the package assembly10 of the present invention. The substrate 16 is covered with a layer ofpolyimide 28 that has a layer of adhesive on both sides of thedielectric material 28. The polyimide 28 covers the areas of thesubstrate 16 that will be adjacent to the signal lines 30 and conductiveplane 32 of the flexible circuit 26. If the substrate 16 is to be usedas a power/ground plane, via holes 46 can be drilled, punched or etchedinto the polyimide 28 before the dielectric is attached to the substrate16.

The device 14 is mounted to the cavity 18 of the substrate 16 bytechniques known in the art. A single flexible circuit 26 containing thesignal lines 30 and conductive plane 32 can then be wrapped around thesubstrate 16, so that the leads 40 of the circuit 26 are aligned withthe surface pads of the device 14. The circuit 26 can then be bonded tothe substrate.16 in an autoclave which applies heat and pressure to theadhesive of the polyimide 28. The leads 40 of the circuit 26 are wirebonded to the pads of the device 14. As an alternate method, the package10 may utilize two separate flexible circuits 26 that are initially wirebonded to the device 14 before the circuit 26 are attached to thesubstrate 16. The wire bonding step of this method may occur before orafter the device 14 is mounted to the substrate 16 . The flexiblecircuit 26 is typically tested before the circuit 26 is bonded to thesubstrate 16.

The substrate 16 may include alignment means such as a pin (s) thataligns the flexible circuit 26 with the substrate 16, so that the leads40 and pads 12 are assembled in the proper locations. The flexiblecircuit 26 can be attached to the top surface 20 of the substrate beforethe circuit 26 is bonded to the device 14, so that the leads 40 remainessentially parallel with the top surface of the device during the wirebonding process. After the device 14 and flexible circuit 26 areattached to the substrate 16, the device 14 is encapsulated with thedielectric material 44.

After the flexible circuit 26 is bonded to the substrate 16, the pads 12can be formed by placing a solder preform 48 into each hole 46 of thecircuit 26. The package is then heated to reflow the solder, so that thesolder attaches to the corresponding conductive members 16, 30 or 32.The solder preforms 48 can be placed into the holes with a simple vacuumcontrolled tooling fixture (not shown). Although the pads 12 are shownand described as being formed after the flexible circuit 26 is assembledto the substrate 16, it is to be understood that the pads 12 could becreated during the manufacturing process of the circuit 26. The assembly10 is typically constructed so that the pads 12 are located throughoutthe entire bottom surface 22 of the substrate 16, thereby maximizing thepin density of the package 10.

While certain exemplary embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive on the broad invention, andthat this invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art.

What is claimed is:
 1. An electronic assembly, comprising:a metalsubstrate having a top surface, a bottom surface and a cavity in saidtop surface; an electrical device mounted into said cavity of saidsubstrate; and, a flexible circuit wrapped around said substrate, saidflexible circuit having a first layer of dielectric that separates saidsubstrate from a plurality of first signal lines and a second layer ofdielectric that separates said first signal lines from a power/groundplane, said first signal lines and said power/ground plane being coupledto said electrical device, said flexible circuit having a plurality ofmetal pads located adjacent to said bottom surface of said substrate andcoupled to said first signal lines and said power/ground plane.
 2. Theassembly as recited in claim 1, wherein at least one of said metal padsis coupled to said metal substrate.
 3. The assembly as recited in claim1, wherein said flexible circuit includes a plurality of second signallines separated from said first signal lines by a layer of dielectric,said flexible circuit having a plurality of vias that couple said firstsignal lines to said second signal lines.
 4. The assembly as recited inclaim 1, further comprising a dielectric material that encapsulates saidelectrical device.
 5. The assembly as recited in claim 1, wherein saidflexible circuit has an outer layer of dielectric that covers saidpower/ground plane.
 6. An electronic assembly, comprising:a metalsubstrate having a top surface, a bottom surface and a cavity in saidtop surface; an electrical device mounted into said cavity of saidsubstrate; and, a flexible circuit wrapped around said substrate, saidflexible circuit having a first layer of dielectric that separates saidsubstrate from a power/ground plane and a second layer of dielectricthat separates said power/ground plane from a plurality of first signallines, said first signal lines and said power/ground plane being coupledto said electrical device, said flexible circuit having a plurality ofmetal pads located adjacent to said bottom surface of said substrate andcoupled to said first signal lines and said power/ground plane.
 7. Theassembly as recited in claim 6, wherein at least one of said metal padsis coupled to said metal substrate.
 8. The assembly as recited in claim7, wherein said flexible circuit includes a plurality of second signallines separated from said first signal lines by a layer of dielectric,said flexible circuit having a plurality of vias that couple said firstsignal lines to said second signal lines.
 9. The assembly as recited inclaim 8, further comprising a dielectric material that encapsulates saidelectrical device.
 10. The assembly as recited in claim 6, wherein saidflexible circuit has an outer layer of dielectric that covers saidplurality of first signal lines.